In general, a vehicle having an internal combustion engine such as a Diesel engine mounted thereon is provided with an exhaust gas purification device configured by a combination of an oxidation catalytic device, a particulate trap device, a lean NOx trap catalytic device, a selective reduction-type catalytic (SCR catalytic) device and the like so as to remove PM (particulate matters), NOx (nitrogen oxide), HC (hydrocarbon) and the like included in an exhaust gas that is to be discharged from the internal combustion engine.
The selective reduction-type catalytic device is configured to perform purification treatment by injecting a urea solution to the exhaust gas from a urea solution injection device provided upstream of the selective reduction-type catalytic device, generating NH3 (ammonia) from the urea solution in the exhaust gas and reducing NOx included in the exhaust gas with NH3.
A melting point of the urea solution is different depending on a concentration of urea but is about −11° C. to 0° C. (urea 0%). In a cold region and the like, when a temperature of the urea solution stored in a urea solution tank becomes equal to or lower than the melting point and the urea solution is thus solidified and frozen, it is difficult to supply the urea solution to the urea solution injection device. Regarding this, for example, as disclosed in a Japanese Unexamined Patent Application Publication No. 2013-217233, the stored urea solution is thawed and the temperature thereof is kept by using engine cooling water or the like.
When starting the NOx treatment in the exhaust gas, it is necessary to rapidly supply the urea solution to the selective reduction-type catalytic device. Therefore, when starting the supply of the urea solution, the urea solution should be at a thawed state. Also, since it is necessary to supply the urea solution to the selective reduction-type catalytic device in an amount corresponding to an amount of NOx in the exhaust gas, a concentration of a urea amount in the urea solution is determined.
In the related art, the flowing of the engine cooling water to warm the urea solution is controlled by opening and closing control of an opening and closing valve on a line of the engine cooling water. However, when an opening timing of the opening and closing valve is slow or a closing timing of the opening and closing valve is fast, the thawing of the urea solution is delayed or insufficient, so that it is not possible to supply the urea solution when the NOx treatment is required. Also, when the closing timing of the opening and closing valve is slow, air bubbles are formed in the urea solution due to an increase in a temperature of the urea solution, so that it is not possible to accurately determine the concentration of urea in the urea solution and to supply a proper amount of the urea solution, which causes a problem in the purification treatment of NOx in the exhaust gas by the exhaust gas purification device.